memory usage in G94

    I'm using Gaussian94 on an SGI Octane.  We recently upgraded to 768MB of
 RAM, with the intention of running large jobs (e.g. CASSCF) incore.  But
 for some reason, I'm unable to use any more than about 512MB for a single
 job.  That is, I can specify up to mem=63000000 in the input file, and it
 will run (though, it seems, more slowly than normal), but if I request more
 than 63 megawords of memory, the job just hangs up.
    Does anyone have any suggestions as to how to overcome this problem, and
 persuade G94 to use more than 63 MW of memory for a job?  Any advice would
 be greatly appreciated.